Conventionally, in a semiconductor device manufacturing process (e.g., a film formation process using a film forming gas, an etching process using an etching gas, and the like), a gas or a liquid is supplied to a semiconductor fabrication apparatus. In this case, a flow rate of the gas or liquid is controlled by a flow rate controller such as a mass flow controller (MFC) installed in a supply flow path through which the gas or liquid is supplied.
The MFC is generally configured to split the supply flow path into a lateral flow path and a main flow path running in parallel. The lateral flow path includes an upstream sensor and a downstream sensor which respectively have heat generating resistance wires. When a fluid is flowing, heat is lost at the upstream to be cooled so that the upstream sensor detects a temperature lowered thereat, while heat is gained at the downstream to thereby be heated so that the downstream sensor detects a temperature raised thereat. As a result, a temperature difference is observed between the upstream sensor and the downstream sensor, and the flow rate in the supply flow path can be obtained by measuring the temperature difference. The MFC adjusts the flow rate in the supply flow path to be a set flow rate, by controlling an opening level of a flow rate control valve in response to outputs from the flow rate sensors.
However, while the MFC is being used, an actual flow rate (i.e., a flow rate of a gas which actually passes through the MFC) may be deviated from the set flow rate, due to contamination (caused by corrosion or remaining products) in a pipe around which the flow rate sensors are wound. For example, even when the actual flow rate is zero, a detected voltage value corresponding to the flow rate detected by the flow rate sensor is in many cases not zero, resulting in an error with slight deviation (see, e.g. Patent Document 1). The above-mentioned deviation of zero point (zero point shift) includes a case where the deviation is slowly increased in proportion to a duration of use, and a case where a change rate (slope) of an output voltage with respect to a flow rate varies (span shift). In the present application, this zero point shift will be referred to as “use-based zero point shift (a first zero point shift)”.
Further, the deviation of the zero point (zero point shift) may also be caused due to, e.g., a molecular weight or pressure of a gas, depending on an installed orientation of the MFC. Due to requirements for a miniaturization of a semiconductor fabrication apparatus, a configuration of a piping system, an installation space or the like, there are cases where a lateral flow path (a part parallel to the main flow path), around which the flow rate sensors of the MFC are wound to be installed, needs to be installed in a vertical state (vertical orientation).
However, when the MFC is installed in the vertical orientation, even though no fluid flows into the supply flow path, for example, a flow may be developed due to the molecular weight or pressure of a gas caused by the temperature difference between the lateral flow path and the main flow path in the MFC. Accordingly, the deviation of the zero point (zero point shift) may occur. This effect is generally referred to as “thermal siphon effect.” (For example, see Patent Document 2.) In the present application, this kind of zero point shift will be referred to as “installation arrangement-based zero point shift (a second zero point shift)”, thereby distinguishing it from the use-based zero point shift (the first second zero point shift) that has been described above. When the second zero point shift occurs, the amount of an actually occurring zero point shift is obtained by adding an amount of the first zero point shift to that of the second zero point shift. In this manner, the zero point shift of the mass flow controller (MFC) may include the second zero point shift as well as the first zero point shift.
(Patent Document 1) Japanese Patent Laid-Open Application No. 2005-38058
(Patent Document 2) Japanese Patent Laid-Open Application No. H11-64060